The present invention relates to improved surface aeration impellers which are used for the surface aeration of liquids in a tank when disposed at the surface of the liquid in the tank, and which have hydraulic performance and adaptations resulting in higher efficiency of aeration. This aeration is particularly important in a number of industrial processes, such as in the aeration of sewage and other wastewater streams. These processes generally involve biochemical oxidation using aerobic microbes. It is typically desirable to transfer oxygen from the surrounding gas or air into the liquid to allow the microbes to work most efficiently.
The two most common techniques for the transfer of oxygen from air or other oxygen containing gas are gas sparging and surface aeration. In a gas sparging procedure, a gas (e.g. air or oxygen) is bubbled through the liquid in a manner that increases the amount of dissolved oxygen in the liquid. In contrast, surface aeration uses an impeller located close to the surface of the liquid to agitate or spray the liquid into the gas. The liquid spray subsequently re-impinges on the liquid surface which also entrains gas into the liquid surface.
Mechanical surface aeration was first introduced more than forty years ago. This technique made use of a mechanical agitator operating near the liquid surface to throw or spray liquid into the air and to induce entrainment of air into the liquid surface, without the use of a compressor and diffusers. Since that time, a fairly large number of different designs for surface aeration impellers have been introduced, both for the purpose of increasing the oxygen-transfer efficiency and also, secondarily, if possible, to improve the bulk liquid mixing and solids suspension. The problem of solids suspension, however, has an obvious limitation because of the remoteness of the surface aeration impeller from the tank bottom where the biomass solids tend to settle if the bulk liquid in the tank is not adequately mixed.
The standard measure of aeration efficiency is the number of pounds of oxygen transferred into the liquid at 20° C. and zero dissolved oxygen level per hour per horsepower used to operate the aeration system. This measure is known as the Standard Aeration Efficiency (SAE). The SAE for current state of the art surface aeration devices ranges from about 2.0 to about 3.3 pounds of oxygen per hour per horsepower in commercial aerator sizes. In smaller sizes, the efficiency values can be somewhat higher. Since wastewater treatment plants are pure cost centers (i.e. they do not sell a product) and since electric power is one of the main operating costs in such a plant, the oxygen-transfer efficiency performance of such aerators is extremely important, especially in larger plants. This need has led to a number of attempts at producing surface aeration impeller designs with greater oxygen transfer efficiency.
Many of the limitations associated with prior art surface aerator impeller designs result from an insufficient understanding of the fundamental mass transfer mechanisms and fluid dynamics of surface aeration. The current state-of-the-art oxygen mass transfer analysis for surface aerators is essentially limited to the simple, idealized model employed in the ASCE Standard for the Measurement of Oxygen Transfer in Clean Water. This oversimplified and limited model has been used for decades to characterize the oxygen mass transfer performance of surface aerators. A more realistic and rigorous mass transfer model has been developed by McWhirter et al. in “Oxygen Mass Transfer Fundamentals of Surface Aerators”, Ind. Eng. Chem. Res. 34, 2644-2654, 1995. This mechanistic model provides a more physically realistic description of the actual oxygen transfer mechanisms of surface aerators and separates the oxygen mass transfer process into two distinct zones: a liquid spray mass transfer zone and a surface reaeration mass transfer zone.
These two distinctly different mass transfer mechanisms or zones are created by all generic types of mechanical surface aerators. The liquid spray mass transfer zone (46 in FIG. 4) is created in the immediate gas space surrounding the periphery of the surface aeration impeller where the liquid is discharged into the surrounding gas at high velocity. The surface reaeration mass transfer zone (48 in FIG. 4) exists primarily outside the spray umbrella and in the bulk liquid near the surface in the area that is circumferential to the periphery of the liquid spray mass transfer zone. The two zones are schematically diagramed in FIG. 4. The liquid spray mass transfer zone can be reasonably characterized and modeled as a single-stage gas-liquid contacting zone wherein the liquid is dispersed into a virtually infinite, continuous gas phase of constant gas composition above the liquid surface. In contrast, the mechanism in the surface reaeration mass transfer zone is predominately characterized by oxygen transfer to a highly turbulent, high velocity liquid phase containing entrained gas from the gas phase above the liquid surface. As the liquid spray zone impinges on the liquid surface of the tank, substantial gas bubble entrainment into the surface is accomplished and a “white-water” effect is produced at the periphery of the liquid spray impingement on the surface of the tank liquid. The surface reaeration mass transfer zone also includes the oxygen transfer to the highly turbulent liquid surface beneath the spray umbrella and thus includes all oxygen transfer to the surface liquid due to bubble entrainment and contact of the highly turbulent liquid surface with the gas above the liquid surface.
In contrast to generally perceived prior opinion regarding the primary oxygen transfer mechanism of surface aerators, the present inventors have quantitatively shown that about two-thirds of the oxygen transfer of surface aerators occurs in the surface reaeration mass transfer zone and only about one-third in the liquid spray mass transfer zone. This suggests that impeller designs that enhance oxygen transfer in the surface reaeration zone (e.g. by increasing surface turbulence and increasing volume flow rates) may have a greater overall effect on the total oxygen transfer of the system than impeller designs that focus primarily on increasing oxygen transfer in the spray zone (e.g. by improving spray characteristics by increasing the height and distance traveled by the sprayed liquid). Thus, a greater understanding of the oxygen mass transfer mechanisms in surface aerators has allowed the present inventors to independently analyze the oxygen transfer process within these two distinctively separate mass transfer zones leading to the improved surface aerator impeller designs as disclosed in this application. These new designs pump more liquid per unit of horsepower input through the liquid spray mass transfer zone and into the surface reaeration zone and thereby maximize the total oxygen mass transfer efficiency of the overall surface aeration system.
Surface aeration impellers which have been used in the past are generally either radial flow impellers or pitched blade turbines (PBT). The blades are flat rectangular plates which are pitched, usually at an angle of 45° to the axis of rotation of the impeller. The 45° pitch is also to the surface of the liquid in the tank when the impeller is not causing flow of the liquid. This is termed the static level of the liquid. Such impellers are located close to the static liquid surface and a small (10 to 20 percent) portion of the width of the blade can project up through the surface. Usually the direction of rotation is such that the leading edge of the blade is above the surface, while the trailing edge is below the surface. In other words, the impeller is pitched forwardly in the direction of rotation of the impeller about its axis of rotation. With such rotation, the impeller is normally down-pumping. The liquid is pushed out in front of the angled blade and discharged radially across the surface of the tank with some of the liquid being sprayed (usually in large drops and not as an atomized spray) into the atmospheric air from the outer upper surfaces of the blade.
Several state of the art surface aeration impellers currently exist, including those shown in U.S. Pat. Nos. 4,066,383 to Lakin; 4,334,826 to Connolly et al; 4,882,098 to Weetman; 5,152,934 to Lally; and 5,988,604 to McWhirter.
Thikotter discloses a surface aeration impeller to be used in an activated sludge process. The aerator comprises a flat, circular impeller disc having a plurality of blades depending from the undersurface of the disc. The blades are generally flat, positioned radially and have a height that decreases from its inner edge to its outer edge. This design primarily focuses on spraying the liquid and does not provide much up-pumping action or mixing of the tank liquid content resulting in relatively low efficiency of the system. In contrast, Lakin and Connolly disclose forms of surface aeration impellers having primarily vertically curved blades. Most seem to have multiple blades on a disc-shaped mounting member.
Both Lally and Weetman teach systems using axial flow impellers which can disperse the gas more efficiently to reduce flooding. McWhirter '604 discloses a surface aeration impeller that is an axial flow impeller that may have either pitched blade turbine or airfoil shaped blades. The blades are not mounted to the underside of a disc, and although the upper section of the blades are not strictly radial, at least at one point the lower section of the blades is radial. However, this impeller still leaves room for improved liquid pumping and oxygen transfer efficiency.
Although these above-described surface aeration impellers have accomplished their purposes, problems remain regarding excessive splashing and misting, insufficient liquid pumping, and overflow of liquid over the surface aerator blades during operation. Thus, there continues to be a need for improved designs that further increase the efficiency of the aeration process.
One problem in particular with some prior art surface aeration impellers is that at the liquid submergence levels of the blades for normal operation as surface aerators, a significant quantity of liquid overflows the upper or leading edge of the blades and falls back into the impeller itself without being pumped and sprayed beyond the outer periphery of the impeller blades. The amount of liquid which is moved per unit of energy input (the hydraulic efficiency) of the impeller is adversely affected due to the flow of liquid over the top of the blade characterizing the normal PBT turbine surface aeration impeller operation. In addition, the overflow of liquid over the leading edge of the blades is believed to overload or flood the impeller with liquid which creates a hydraulic condition detracting from its hydraulic pumping capacity and oxygen transfer efficiency.
A surface aeration impeller provided by the present invention has a structure and mode of operation which counteracts the foregoing hydraulic and oxygen transfer deficiencies.
Therefore, it is the principal object and feature of this invention to provide improved surface aeration impellers which are especially adapted for use as surface aerators which operate more efficiently than conventional surface aeration impellers, and particularly by better controlling the flow of liquid and spray.
It is a further object of the invention to provide improved axial flow aeration impellers which may be operated in an up-pumping direction causing flow, which creates a hydraulic surge ahead of and radially outward from the impeller at a plurality of positions radially outward in the tank, at each of which increased turbulence occurs, such as splashing, which further enhances the oxygen transfer efficiency of the system.
It is a still further object of the present invention to provide improved PBT aeration impellers.
It is a still further object of the present invention to provide improved surface aeration impellers which may have camber and may be of air foil shape.